Update historic run script to use main instead of master

Change-Id: I1e9b8f0b21388c43e5e754de5d1085ec2465367d
diff --git a/tools/historic_run.py b/tools/historic_run.py
index 9a035b0..3b17903 100755
--- a/tools/historic_run.py
+++ b/tools/historic_run.py
@@ -15,7 +15,7 @@
 import time
 import utils
 
-MASTER_COMMITS = 'gs://r8-releases/raw/master'
+MASTER_COMMITS = 'gs://r8-releases/raw/main'
 
 
 def ParseOptions(argv):